Two apical membrane proteins in intestinal epithelial cells are important effectors of cAMP-mediated signaling that result in secretory diarrheas: NHE-3 and CFTR. Both are attached via PDZ interactions to NHERF proteins, which, in turn, are attached to ezrin and actin. The integrity of this scaffold is essential for PKA/cAMP signaling on CFTR and may play a role as an apical retention signal for polarization. Previous published work in our lab has highlighted the importance of intermediate filaments (IPs) in the organization of the apical domain in simple epithelial cells, and suggest that IPs play a role in the organization of the apical actin-based scaffold in these cells. Furthermore, IPs mediate the assembly of ezrin into the above-mentioned scaffold, and PKCiota or alpha may be responsible for the activation of ezrin. The hypothesis is that binding of ezrin to apical intermediate filaments initiates the apical localization of ezrin in intestinal cells and provides a microdomain-restricted site for its activation by PKCi. The assembly of this ezrin-based scaffold is essential for the function of the effectors of secretory diarrheas (NHE-3 and CFTR). To test this hypothesis I propose to: 1) Identify the molecular interactions between ezrin and keratins and their role to localize ezrin to the apical domain of enterocytes, using T567D / T567A ezrin mutants in vitro binding assays, or GFP-ezrin constructs in vivo. 2) Identify the physiological activator of ezrin in the brushborder and the molecular interactions between PKCi or a and keratins, using recombinant PKCisoforms, expression of anti- PKCi shRNA, dominant negative PKCisoforms, pharmacological blockers, stable TET-inducible CACO-2 cell lines that knock down keratin 8, and K8 null mice. And 3) Test the functional consequences of apical ezrin complex assembly on cAMP-dependent CI"secretion, PKA localization and activation, signaling downstream of ezrin and CFTR localization, using the experimental variables and reagents created and analyzed in the previous aims in Ussing chamber experiments. The long- term goal of this project is to identify the molecular interactions responsible for the assembly of the apical ezrin- actin scaffold in intestinal cells that can be manipulated to interferewith the function of ion channels in diarrheal disorders. Lay statement: Secretory diarrheas are a severe health problem in the U.S. and worldwide. All of them operate through common effectors in the intestine that enable the secretion of chloride (and water). The machinery that supports these membrane proteins is based on cytoskeletal components and is assembled around a protein known as ezrin. This project seeks to test the hypothesisthat ezrin (and its associated chloride secreting proteins) require spatial cues from a division of the cytoskeleton,the intermediate filaments, to become assembled in the precise location to secrete water to the lumen of the intestine. The study of protein-protein interactions that result in the precise and adequate localization of ezrin and its associated proteins will provide potential points for molecular intervention on the function of this molecular complex.